Curiosity regarding surging aurothrottles/porpoising pitch at cruise

I am flying a 767-400 from KSFO to KJFK @ FL350 275 KIAS on X-Plane 10. I am traveling 78° mag towards BCE VORTAC with a 22-32 knot wind @ 133°. Why am I noticing my throttles surge and a 3° pitch up/down (total range of 6°)? If I slow down to 275 KIAS, the porpoising stops and so does the surging.

For the most part, I understand flight dynamics due to my avionics maintenance background but my training doesn't aid me with this common problem with multiple heavy aircraft. My engines are already working hard as it is at 100%+ N1. I know this aircraft can handle the loads I'm putting on it too. Advice please?

Bear in mind that above 18,000' MSL you should be giving IAS in Mach numbers. IE: Just WHAT Mach number is 275kts at FL350? What %Mach number is the plane supposed to cruise at? Climb at? What %N1 is it supposed to cruise at? And how does "slowing" from 275KIAS to 275KIAS help? A typo on your part, I am sure, but...

Some plane autopilot PID systems aren't very well tuned, so you can get the porpoising effect from the ALT HOLD system. Same for the auto-throttle.

Or perhaps you were trying to fly too fast for your given altitude/weight profile. Does it still do it at a lower weight, IE once you've burned off some fuel, but at the same altitude? At the same weight at lower altitudes? Higher?

Lotta things to try :D

There are also some aircraft.cfg settings that will change this sort of behaviour, but PID systems can be twitchy and hard to adjust. Change one thing a little too much, and the whole thing fails, so...

Pat☺

At 275 Kias @FL350 you are simply going too slowly for the thin air. I am assuming you are under auotpiolot and it is your autopilot that is causing this surge - you are likely close to stall speed so the aeroplane pitches up (no idea why saw it on aircrash investigations once) then the autopilot pitches it down to increase airflow speed and to stop the stall then this repeats over and over.

As the above poster stated above FL180 you should switch to mach mode, although I am not certain the mach speed for your aeroplane a quick search suggests it should be travelling at mach 0.80 at FL350. This is somewhere equivalent to 475 Knots. At 275 KIAS you are probably closer to 310 knots quite a bit slower than you should be to achieve stable flight.

This is somewhere equivalent to 475 Knots. At 275 KIAS you are probably closer to 310 knots quite a bit slower than you should be to achieve stable flight.

To be a bit more precise, I believe Delphi is meaning TAS (true air speed) when he says "475 Knots", which is usually abbreviated KTAS, as opposed to KIAS which is Knots Indicated Air Speed (what you see on your gauges in the cockpit). Basically, as I understand things, and it's FAR from perfect, KTAS is the indicated airspeed adjusted to sea level pressure (zero feet in altitude), on a "standard day". And all this is as opposed to ground speed, which is how fast the plane crosses the ground, counting in winds, and so on. Of corse, then there is always % Mach number, which is yet another way of expressing KIAS, or the speed the plane's Air Data Computers see when moving through the air.

There are add-on gauges, sually made for pop-up windows that can and will display any and all of the mentioned methods of expressing airspeed, groundspeed, Mach, and so on, so if you want you can cross-check all of them for gauge accuracy, autopilot setting correctness, etc etc etc.

Not sure if my petty quibbling matters. It's a bad habit I have...

Pat☺